CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.

1. NATURAL SELECTION

How will the struggle for existence, briefly discussed in the last chapter,
act in regard to variation? Can the principle of selection, which we have
seen is so potent in the hands of man, apply under nature? I think we
shall see that it can act most efficiently. Let the endless number of
slight variations and individual differences occurring in our domestic
productions, and, in a lesser degree, in those under nature, be borne in
mind; as well as the strength of the hereditary tendency. Under
domestication, it may truly be said that the whole organisation becomes in
some degree plastic. But the variability, which we almost universally meet
with in our domestic productions is not directly produced, as Hooker and
Asa Gray have well remarked, by man; he can neither originate varieties nor
prevent their occurrence; he can only preserve and accumulate such as do
occur. Unintentionally he exposes organic beings to new and changing
conditions of life, and variability ensues; but similar changes of
conditions might and do occur under nature. Let it also be borne in mind
how infinitely complex and close-fitting are the mutual relations of all
organic beings to each other and to their physical conditions of life; and
consequently what infinitely varied diversities of structure might be of
use to each being under changing conditions of life. Can it then be
thought improbable, seeing that variations useful to man have undoubtedly
occurred, that other variations useful in some way to each being in the
great and complex battle of life, should occur in the course of many
successive generations? If such do occur, can we doubt (remembering that
many more individuals are born than can possibly survive) that individuals
having any advantage, however slight, over others, would have the best
chance of surviving and procreating their kind? On the other hand, we may
feel sure that any variation in the least degree injurious would be rigidly
destroyed. This preservation of favourable individual differences and
variations, and the destruction of those which are injurious, I have called
Natural Selection, or the Survival of the Fittest. Variations neither
useful nor injurious would not be affected by natural selection, and would
be left either a fluctuating element, as perhaps we see in certain
polymorphic species, or would ultimately become fixed, owing to the nature
of the organism and the nature of the conditions.